1. Field of the Invention
The present invention relates to a television signal converter for converting television signal input of an arbitrary channel into television signal output of a predetermined channel. More specifically, it relates to an improvement in a signal converter which receives a television signal such as a CATV signal formed by a scrambled video signal to convert the same into a descrambled television signal of a predetermined channel.
2. Description of the Prior
In countries such as U.S. and Canada, subscription television broadcasting is popularly made through developed CATV systems. A CATV system is adapted to transmit television signals of a plurality of channels from a broadcasting station to subscribers through cables such as coaxial cables. Within the transmitted television signals, those of premium programming are scrambled to prevent unauthorized viewing of the programming. Authorized subscribers for the premium programming, i.e., subscribing addressees, can descramble the scrambled television signals to watch the programming. Such a system of subscription television broadcasting is generally called a "pay per view system".
Scrambling in the CATV system or the like is generally performed by a method of compressing the amplitude of a carrier accompanied by a video signal (hereinafter referred to as "video carrier" in this specification) to convert the same, through a system such as a timing-tag system. Details of the scrambling through the timing-tag system are disclosed in, e.g., U.S. Continued Pat. No. 4,471,380 and Japanese Patent Laying-Open Gazette No. 51678/1983. In the timing-tag system, the video carrier is compressed by a prescribed level in intervals of a horizontal synchronizing signal part to be scrambled. At the same time, a frequency-modulated audio signal carrier, i.e., a carrier accompanied by an audio signal (hereinafter referred to as "audio carrier" in this specification) is amplitude-modulated by a timing-tag signal which is synchronous with the horizontal synchronizing signal part, to frequency-multiplex and transmit the amplitude-modulated audio carrier and the scrambled video carrier.
For example, U.S. Continued Pat. No. 4,323,922 discloses an example of a conventional signal converter which is applied to a receiver for the television signal scrambled through the aforementioned timing-tag system. In the conventional signal converter as disclosed in this Patent, however, the audio carrier amplitude-modulated by the timing-tag signal is influenced by gain changing following descrambling, whereby the reproduced audio signal includes noise or the like.